github: Upgrade checkout@v4 to checkout@v6

checkout@v5 should be the same as checkout@v4, just running on
Node 24. checkout@v6 has got a few other minor changes that
shouldn't make any difference; opt in to newer behaviours now.

.github/workflows/release.yml

@@ -52,7 +52,7 @@ jobs:
       - name: Set environment
         run: |
           cat parameters/parameters.txt >> $GITHUB_ENV
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           ref: ${{inputs.ref}}
           path: repo